1. Field of Invention
The invention relates to methods for preparing single walled carbon nanotubes. More specifically, the invention relates to methods for preparing a bundle or a densely packed array of single walled carbon nanotubes under commercially viable reaction conditions.
2. Description of the Related Art
Carbon Nanotubes
This invention lies in the field of carbon nanotubes (also known as fibrils). Carbon nanotubes are vermicular carbon deposits having diameters less than 1.0μ, preferably less than 0.5μ, and even more preferably less than 0.2μ. Carbon nanotubes can be either multi walled (i.e., have more than one graphene layer more or less parallel to the nanotube axis) or single walled (i.e., have only a single graphene layer parallel to the nanotube axis). Other types of carbon nanotubes are also known, such as fishbone fibrils (e.g., wherein the graphene layers are arranged in a herringbone pattern, compared to the tube axis), etc. As produced, carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes (i.e., dense, microscopic particulate structure comprising entangled carbon nanotubes) or a mixture of both.
Carbon nanotubes are distinguishable from commercially available continuous carbon fibers. For instance, diameter of continuous carbon fibers, which is always greater than 1.0μ and typically 5 to 7μ, is far larger than that of carbon nanotubes, which is usually less than 1.0μ. Carbon nanotubes also have vastly superior strength and conductivity than carbon fibers.
Carbon nanotubes also differ physically and chemically from other forms of carbon such as standard graphite and carbon black. Standard graphite, because of its structure, can undergo oxidation to almost complete saturation. Moreover, carbon black is an amorphous carbon generally in the form of spheroidal particles having a graphene structure, such as carbon layers around a disordered nucleus. On the other hand, carbon nanotubes have one or more layers of ordered graphitic carbon atoms disposed substantially concentrically about the cylindrical axis of the nanotube. These differences, among others, make graphite and carbon black poor predictors of carbon nanotube chemistry.
It has been further accepted that multi walled and single walled carbon nanotubes are also different from each other. For example, multi walled carbon nanotubes have multiple layers of graphite along the nanotube axis while single walled carbon nanotubes only have a single graphitic layer on the nanotube axis.
The methods of producing multi walled carbon nanotubes also differ from the methods used to produce single walled carbon nanotubes. Specifically, different combinations of catalysts, catalyst supports, raw materials and reaction conditions are required to yield multi walled versus single walled carbon nanotubes. Certain combinations will also yield a mixture of multi walled and single walled carbon nanotubes.
As such, two characteristics are often examined in order to determine whether such process will be commercially feasible for the production of a desired carbon nanotube on an industrial scale. The first is catalyst selectivity (e.g., will the catalyst yield primarily single wall carbon nanotubes or primarily multi-walled carbon nanotubes or other forms of carbon products?). The second is catalyst yield (e.g., weight of carbon product generated per weight of catalyst used).
Processes for forming multi walled carbon nanotubes are well known. E.g., Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993); Oberlin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp. 335-349; U.S. Pat. No. 4,663,230 to Tennent; U.S. Pat. No. 5,171,560 to Tennent; Iijima, Nature 354, 56, 1991; Weaver, Science 265, 1994; de Heer, Walt A., “Nanotubes and the Pursuit of Applications,” MRS Bulletin, April, 2004; etc. All of these references are herein incorporated by reference.
Commercially known processes for forming multi walled carbon nanotubes are high in selectively (e.g., produces greater than 90% multi walled carbon nanotubes in product) as well as yield (e.g., produces 30 pounds of multi walled carbon nanotube produce per pound catalyst).
Processes for making single walled carbon nanotubes are also known. E.g., “Single-shell carbon nanotubes of 1-nm diameter”, S Iijima and T Ichihashi Nature, vol. 363, p. 603 (1993); “Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls,” D S Bethune, C H Kiang, M S DeVries, G Gorman, R Savoy and R Beyers Nature, vol. 363, p. 605 (1993); U.S. Pat. No. 5,424,054 to Bethune et al.; Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smally, R. E., Chem. Phys. Lett. 243: 1-12 (1995); Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996); Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 260: 471-475 (1996); U.S. Pat. No. 6,761,870 (also WO 00/26138) to Smalley, et. al; “Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co—Mo catalysts,” Chemical Physics Letters, 317 (2000) 497-503; U.S. Pat. No. 6,333,016 to Resasco, et. al., etc. All of these references are hereby by reference.
However, unlike multi walled carbon nanotube technology, currently known processes for forming single walled carbon typically are unable to reach industrially acceptable levels of selectivity and yield under commercially viable reaction conditions. For example, in Maruyama, et. al. “Low-temperature synthesis of high-purity single walled carbon nanotubes from alcohol,” Chemical Physics Letters, 360, pp. 229-234 (Jul. 10, 2002), herein incorporated by reference, a method is disclosed for obtaining high purity single walled carbon nanotubes under vacuum or extremely low pressure (e.g., 5 Torr). Maintaining such extremely low pressure conditions on an industrial scale reactor would not be commercially viable. Other references such as U.S. Pat. No. 6,333,016 to Resasco also disclose high selectivity for single walled carbon nanotubes, but fail to show a commercially viable yield.
As such, there is a need for a method for producing single walled carbon nanotubes with industrially acceptable levels of activity, selectivity and yield under commercially viable reaction conditions.